Methods and apparatus for material processing using atmospheric thermal plasma reactor

ABSTRACT

Methods and apparatus provide for: producing a plasma plume within a plasma containment vessel from a source of plasma gas; feeding an elongate feedstock material having a longitudinal axis into the plasma containment vessel such that at least a distal end of the feedstock material is heated within the plasma plume; and spinning the feedstock material about the longitudinal axis as the distal end of the feedstock material advances into the plasma plume, where the feedstock material is a mixture of compounds that have been mixed, formed into the elongate shape, and at least partially sintered.

BACKGROUND

The present disclosure relates to methods and apparatus for materialprocessing using a atmospheric thermal plasma reactor.

Glass substrates may be used in a variety of applications, includingwindows, high-performance display devices, and any number of otherapplications. The quality requirements for glass substrates have becomemore stringent as the demand for improved resolution, clarity, andperformance increases. Glass quality may, however, be negativelyimpacted by various processing steps, from forming the glass melt tofinal packaging of the glass product.

One processing step that may result in reduced glass quality is themelting process, wherein the components of a glass batch material aremixed and heated in a melting apparatus. During this process, thecomponents of the glass batch material melt and react, giving offreaction gases, which produce bubbles in the molten glass. Additionally,the melting process may produce an inhomogeneous glass melt havingregions of differing chemical compositions. The first melt to form isoften highly reactive with the refractory materials, which may lead toexcessive wear of the apparatus and/or defects in the glass melt. Denserportions of the melt may also sink to the bottom of the meltingapparatus, leading to a sludge layer, which has different opticalproperties than the rest of the melt and is difficult to completely mixback into the overall melt. The sludge layer therefore results ininhomogeneous portions of the melt, referred to in the art and herein aschord. Finally, due to typically large processing volumes, it ispossible that various glass batch materials may not completely melt. Anyun-melted or partially melted materials are carried through the meltingprocess and may later become defects in the glass product.

Current melting processes for producing high quality optical glassutilize high temperatures and stirring to remove bubbles from the glassmelt. However, such processes may be cost prohibitive, as they requireexpensive metals and specially designed high temperature refractorymaterials for the processing equipment. Further, these costly meltingsystems require a long processing time and high energy expenditure asthe reaction gases have a long distance to travel to escape the glassmelt and the sludge layer must be mixed from the bottom of the meltertank into the rest of the glass melt in the tank, requiring a mixingmotion over a long distance through a highly viscous fluid.

Alternative methods for preventing glass bubbles and inhomogeneousportions in the glass melt include processing the melt in smallerbatches. In this manner, the gas bubbles have a shorter distance totravel to escape the melt and the sludge layer can be more easilyincorporated into the rest of the melt. However, as with many smallscale processes, these methods have various drawbacks such as increasedprocessing time and expense.

Accordingly, there are needs in the art for techniques to improve themelting processes of glass batch material for producing high qualityoptical glass.

SUMMARY

The present disclosure relates to an area of material processing (forexample, glass batch material) by means of atmospheric thermal plasma inwhich the material to be processed is dispensed as material feedstock(containing partially sintered material particles) into a plasma plumethat is of a generally cylindrical configuration. For commercialpurposes, it is important that the atmospheric thermal plasma processexhibits high throughput and sufficient thermal energy to achieve thedesired thermal reaction.

Inductively coupled plasma (ICP) systems have been used for low pressuresputtering and etching systems on substrates. Inductively coupledatmospheric plasma material processing systems are generally constructedwith small diameter coils or microwave waveguides which limit the plasmato a small volumetric column (typically about 5 mm in diameter). Even ifsuch a system employs a relatively high power RF source (e.g., about 400kW), at a very high equipment cost, only a low rate (e.g., 20-40 kg perhour) of particulate material may be processed through the plasma. Inthe glass batch processing context, practical production rates are atleast one metric ton per day, which would barely be met using theconventional ICP system at peak production twenty four hours a day. Inorder to address the shortcomings of the processing rate, multiples ofthe equipment set up, energy, and maintenance costs would be required.

Another problem with the conventional ICP system is a limit on thepermissible input particle sizes, typically about 90 um or less. Thefree fall characteristics of such small particles in the ICP plasmasystem are such that sufficient heating of the particles may be achievedwithin a period of about 300 ms or less. If the particles were larger,and did not absorb enough heat to melt, then the once through-processedparticles would have to be recycled through the system again, therebyreducing the throughput rate even further.

One or more embodiments disclosed herein provide a new material feedcapability in a plasma containment vessel to thermally process thematerial. In the context of glass batch material processing, thecompounds of the glass batch material are mixed to provide a homogeneousdistribution of the compounds. Then the glass batch material is pressedand partially sintered to hold its shape as a feedstock, such as agenerally cylindrical rod form. The feedstock is continuously insertedinto a plasma containment vessel and the feedstock is rotated within thecenter of a plasma plume within the plasma containment vessel. Notably,this new approach for material introduction avoids at least some of theissues with conventional plasma processing because there is no need tointroduce separate granulated powder into the plasma plume. As a distalend of the feedstock absorbs energy from the plasma plume, the feedstockmelts and droplets of molten material (in this example glass material)are formed into spheres and flung from the feedstock due to centrifugalforces. The reactive gases boil off of the respective spheres ofmaterial. The liquid spheres are then rapidly quenched and collected orfed into a next processing stage (e.g., a pre-melter or the like). Thesize distribution of the droplets is determined by the rotational speedof the feedstock within the thermal environment of the plasma plume.

The embodiments disclosed herein overcome the low particulate materialprocessing rates of existing systems in order to provide industrialscale applications. The embodiments provide high volumes of plasma atatmospheric pressures, and produce adequate kinetic energy within theplasma plume to heat the material and achieve desired reactions,including melting and/or other thermally-based processes.

Other aspects, features, and advantages will be apparent to one skilledin the art from the description herein taken in conjunction with theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS

For the purposes of illustration, there are forms shown in the drawingsthat are presently preferred, it being understood, however, that theembodiments disclosed and described herein are not limited to theprecise arrangements and instrumentalities shown.

FIG. 1 is a schematic illustration of a length of feedstock material,such as glass batch material, according to one or more embodiments ofthe present disclosure;

FIG. 2 is flow diagram illustrating processing steps for producing thefeedstock material of FIG. 1 according to one or more embodiments of thepresent disclosure;

FIG. 3 is a side, schematic illustration of one or more embodiments ofan apparatus for carrying out a process for producing the feedstockmaterial of FIG. 1 according to one or more embodiments of the presentdisclosure; and

FIG. 4 is a side, schematic illustration of one or more embodiments of aplasma containment vessel usable for thermally processing the feedstockmaterial of FIG. 1 and/or other embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings wherein like numerals indicate likeelements there is shown in FIG. 1 a feedstock material 10 produced inorder to be thermally treated within a plasma thermal source. Thefeedstock material 10 may be composed of any suitable material, forexample glass batch material. Although particular embodiments herein maybe presented in the context of thermally treating glass batch material,the embodiments herein are not limited to glass batch material.

The feedstock material 10 denotes a mixture of precursor compoundsand/or particles which, upon melting, reacting and/or other action,combine to form a particular, desired material. In the case of glassbatch material, the precursor compounds may include silica, alumina, andvarious additional oxides, such as boron, magnesium, calcium, sodium,strontium, tin, or titanium oxides. For instance, the glass batchmaterial may be a mixture of silica and/or alumina with one or moreadditional oxides. One skilled in the art will appreciate that glassbatch material may take on a wide variety of specific combinations ofcompounds and substances.

With reference to FIGS. 1-3, the feedstock material 10 contains aplurality of precursor compounds that have been mixed, formed into anelongate shape, and at least partially sintered. FIG. 2 is a flowdiagram illustrating processing steps for producing the feedstockmaterial 10 and FIG. 3 is a side, schematic illustration of one or moreembodiments of a feedstock processing mechanism 300 for carrying out aprocess for producing an extruded source of the feedstock material 10.At step 150 of the process flow, the plurality of precursor compoundsare mixed. By way of example, the precursor compounds may be directed toproducing glass and contain one or more of SiO₂, Al₂O₃, B₂O₃, MgO,CaCO₃, SrCO₃, SnO₂ and/or various mixtures thereof. The precursorcompounds may be mixed in a batch feeder, mixer, and/or batch sifter302, where the precursor compounds are thoroughly mixed and anyagglomerates are broken up (step 152).

The mixed precursor compounds may be fed into a powder tray 304, whichfunnels the mixed precursor compounds into a rotating powder die 306. Apowder ram 308 operates in conjunction with the powder die 306 in orderto apply pressure to the mixed precursor compounds and to shape themixed precursor compounds into an elongate shape (step 154). Acompression force of compaction may be from about 20 psi to 200 psi.

The pressed precursor compounds are next heated in order to at leastpartially sinter the precursor compounds into the feedstock material 10(step 156). By way of example, the feedstock processing mechanism 300may include an inductive heating mechanism 310 comprising a coil 312about a central axis. The coil 312 may be wound about a graphitesuscepter 314 through which the pressed precursor compounds pass.Activation of the coil 312 causes the graphite suscepter 314 to heat up,which in turn heats the pressed precursor compounds as such materialpasses through the graphite suscepter 314 (and the coil 312) along thecentral axis thereof. The heating is controlled in order to achieve atleast partial sintering of the pressed precursor compounds. For example,an inductive heating mechanism 310 may operate to heat the pressedprecursor compounds to between about 500-1000° C. This may be achievedby applying an AC power source to the coil 312 of sufficient magnitude,such as from about 10 kW to 500 kW (depending on the desired materialthroughput). A frequency of the AC power provided to the inductiveheating mechanism 310 (i.e., to the coil 312) may range from about 50kHz to 500 kHz.

The parameters of the mixing, sifting, pressing, and/or heating may beadjusted in order to attain a feedstock material 10 of desired diameter,mechanical strength, and/or thermal reactivity. For example, thefeedstock processing mechanism 300 may be adjusted to produce afeedstock material 10 having a diameter of one of: (i) between about 5mm-50 mm; (ii) between about 10 mm-40 mm; and (iii) between about 20mm-30 mm.

The extruded feedstock material 10 may be produced beforehand and storedfor later use in a plasma reactor (step 158), or the feedstockprocessing mechanism 300 may be integrated with the plasma reactor suchthat the extruded feedstock 10 may be fed in a continuous process intothe plasma reactor.

Reference is now made to FIG. 4, which is a side, schematic illustrationof one or more embodiments of a plasma containment vessel 200 usable forthermally processing the feedstock material 10 of FIGS. 1-3 and/or otherembodiments. The plasma containment vessel 200 includes at least onewall member 202 defining an inner volume 218 having a central axis, aninlet end 204, and an opposing outlet end 206. The at least one wallmember 202 may be formed from a suitable non-conductive, non-corrosive,high temperature, dielectric material, such as high temperature ceramicmaterials, quartz, preferably with an ultra-low coefficient of thermalexpansion. In order to permit cooling of one or more components of theplasma containment vessel 200, the wall member 202 may include one ormore internal channels operating to carry cooling fluid therethrough. Inthis regard, the internal channels may be accessed via respectiveinlet/outlets that are in fluid communication with the one or moresources of cooling fluid (not shown).

The plasma containment vessel 200 may include a mechanism configured toreceive a source of RF power (not shown) having characteristicssufficient to produce an electromagnetic field within the plasmacontainment vessel 200 for maintaining a plasma plume 220 from a sourceof plasma gas (not shown). For example, the mechanism may include aninduction coil 210 disposed about the central axis of the plasmacontainment vessel 200, and the coil 210 may be operable to receive thesource of RF power and produce the electromagnetic field. By way ofexample, the RF power may be of a characteristic such that theelectromagnetic field exhibits a frequency of at least one of: (i) atleast 1 MHz, (ii) at least 3 MHz, (iii) at least 4 MHz, (iv) at least 5MHz, (v) at least 10 MHz, (vi) at least 15 MHz, (vii) at least 20 MHz,(viii) at least 30 MHz, (ix) at least 40 MHz, and (x) between about 1 to50 MHz. The RF power may be at a power level from about 5 kW to 1 MW (orother suitable power level).

A material inlet 250 may be disposed at the inlet end 204 of the plasmacontainment vessel 200, where the material inlet 250 may operate toreceive the elongate feedstock material 10. Thus, the feedstock material10 is introduced into the plasma containment vessel 200, where a distalend 12 of the feedstock 10 encounters the plasma plume 220. The plasmaplume 220 is of sufficient thermal energy to cause at least a thermalreaction of the feedstock material 10. In particular, the plasma plume220 may be of a substantially cylindrical shape, and may be ofsufficient thermal energy, to cause the distal end 12 of the feedstockmaterial 10 to melt, thereby producing respective substantiallyspherical droplets 14.

By way of example, the plasma containment vessel 200 may further includea rotation assembly 252 disposed in communication with the materialinlet 250 and operating to permit the feedstock material 10 to spinabout the longitudinal axis as the distal end 12 of the feedstockmaterial 10 advances into the plasma plume 220. The rotation assembly252 may be operable to spin the feedstock material 10 about thelongitudinal axis at a sufficient speed to cause the melt to separatefrom the distal end 12 of the feedstock material 10, in response tocentrifugal force, and to form the substantially spherical droplets 14.The rotational assembly 252 may include a feed tube 254 in coaxialorientation with a bearing assembly 256 (such as a ball bearingarrangement), which permits the feedstock material 10 to be guidedwithin, and rotated by, the feed tube 254.

A controller (such a microprocessor controlled mechanism, not shown) mayoperate to control the rotation assembly 252 in order to vary a rate atwhich the feedstock material 10 spins, thereby controlling a size of thedroplets 14. BY way of example, the rotation assembly 252 may spin thefeedstock material 10 at a rate of one of: (i) between about 500rpm-50,000 rpm; (ii) between about 1000 rpm-40,000 rpm; (iii) betweenabout 1400 rpm-30,000 rpm; (iv) between about 2000 rpm-20,000 rpm; and(v) between about 5000 rpm-10,000 rpm. These spin rates may producedroplets having a size of one of: (i) between about 10 um-5000 um; (ii)between about 50 um-2000 um; (iii) between about 100 um-1000 um; (iv)between about 50 um-200 um; and (v) about 100 um.

It is noted that the size of the droplets 14 may also be affected by atemperature of the plasma plume 220. In accordance with one or moreembodiments, a controller (not shown) may operate to control a powerlevel of the RF power, thereby controlling an intensity of theelectromagnetic field within the plasma containment vessel 200 and atemperature of the plasma plume 220. By way of example, the plasma plumemay have a temperature ranging from one of: (i) about 9,000 K to about18,000 K; (ii) about 11,000 K to about 15,000 K; and (iii) at leastabout 11,000 K.

The plasma plume is preferably of sufficient thermal energy to cause thedroplets 14 from the feedstock material to thermally react. Examples ofthe types of thermal reactions contemplated herein include, at least oneof: (i) at least partially melting the droplets 14 of material, (ii) atleast partially melting at least one of the droplets 14 of material andone or more further materials thereby forming coated material particles,and (iii) at least partially melting the droplets 14 of material to formsubstantially homogeneous, spheroid-shaped intermediate particles.

Those skilled in the art will appreciate that the types of thermalreactions (and/or other reactions) within the plasma containment vessel200 may include any number of additional reactions as would be evidentfrom the state of the art. By way of example, the feedstock material maybe at least partially melted with a further material that comprisessilver, copper, tin, silicon or another semiconductor material,including the respective metal or metal oxide, etc. to form coated glassbatch material particles. Glass particles coated with silver or copper,for instance, may have antibacterial properties, and glass particlescoated with tin oxide may be photoactive.

The thermally reacted material is accumulated in a collection vessel170. After collection, the thermally reacted material may be subjectedto additional and/or optional processing steps.

The conventional approaches to prepare batch material, for example tomake glass via a plasma process, requires special steps in order toreduce or eliminate fining and stirring. These steps may include amixing step and a spray-drying step for a binding operation to produceagglomerates of the appropriate size to allow plasma energy absorptionas the particles drop through the plasma. In accordance with theembodiments herein, however, such preparation and particle selection isnot necessary since the precursor compounds are mixed to provide evendistribution of the compounds throughout the batch, and the precursorcompounds are pressed and partially sintered into a rod to be fed intothe plasma plume. This mechanism permits a continuous feed process at ahigher throughput without the aforementioned, complex preparationprocedure. Therefore, specific selection of particle sizes (e.g., <90um) are not required for plasma processing. Further, spray-drying forbinding and producing agglomerates are not required for plasmaprocessing. Still further, multiple recycling of material for additionalplasma processing is not required. Indeed, high material throughput isachieved since the compacted rod of batch material with high bulkdensity is processed (as opposed to isolated individual particles),where the droplet production rate is significantly higher than inconventional plasma systems processing powder. The embodiments hereinalso provide reactive gas dissipation before glass particles are placedin a pre-melter, which reduces the need for fining. In addition,homogenization of the precursor compounds in the extrusion yields auniform glass density in the glass particles prior to inclusion in thepremelter reducing the need for stirring.

Although the disclosure herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of theembodiments herein. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present application.

1-13. (canceled)
 14. A method, comprising: producing a plasma plumewithin a plasma containment vessel from a source of plasma gas; feedingan elongate feedstock material having a longitudinal axis into theplasma containment vessel such that at least a distal end of thefeedstock material is heated within the plasma plume; and spinning thefeedstock material about the longitudinal axis as the distal end of thefeedstock material advances into the plasma plume, wherein the feedstockmaterial is a mixture of compounds that have been mixed, formed into theelongate shape, and at least partially sintered.
 15. The method of claim14, wherein the feedstock material is a glass batch material.
 16. Themethod of claim 14, wherein the plasma plume is of a substantiallycylindrical shape, and is of sufficient thermal energy to cause thedistal end of the feedstock material to melt.
 17. The method of claim16, wherein step of spinning the feedstock material about thelongitudinal axis includes spinning at a sufficient speed to cause themelt to separate from the distal end of the feedstock material, inresponse to centrifugal force, and to form respective substantiallyspherical droplets.
 18. The method of claim 17, further comprisingcontrolling a rate at which the feedstock material spins, therebycontrolling a size of the droplets.
 19. The method of claim 17, whereinthe size of the droplets is one of: (i) between about 10 um-5000 um;(ii) between about 50 um-2000 um; (iii) between about 100 um-1000 um;(iv) between about 50 um-200 um; and (v) about 100 um.
 20. The method ofclaim 14, wherein the rate of spinning the feedstock material is one of:(i) between about 500 rpm-50,000 rpm; (ii) between about 1000 rpm-40,000rpm; (iii) between about 1400 rpm-30,000 rpm; (iv) between about 2000rpm-20,000 rpm; and (v) between about 5000 rpm-10,000 rpm.
 21. Themethod of claim 14, further comprising controlling a temperature of theplasma plume thereby controlling a size of the droplets.
 22. The methodof claim 21, wherein the plasma plume has a temperature ranging from oneof: (i) about 9,000 K to about 18,000 K; (ii) about 11,000 K to about15,000 K; and (iii) at least about 11,000 K.
 23. The method of claim 17,wherein the plasma plume is of sufficient thermal energy to cause thedroplets from the feedstock material to thermally react.
 24. The methodof claim 23, wherein at least one of: the thermal reaction includes atleast partially melting the droplets of material, the thermal reactionincludes at least partially melting at least one of the droplets ofmaterial and one or more further materials thereby forming coateddroplets of material, and the thermal reaction includes at leastpartially melting the droplets of material to form substantiallyhomogeneous, spheroid-shaped intermediate particles.
 25. The method ofclaim 14, further comprising producing the feedstock material bysubstantially continuously receiving, mixing, pressing, and at leastpartially sintering precursor compounds into the feedstock material asthe feedstock material is fed into the plasma containment vessel. 26.The method of claim 25, wherein the pressing step includes pressing themixed precursor compounds to between about 20 psi-200 psi.
 27. Themethod of claim 25, wherein the heating step includes heating thepressed precursor compounds to between about 500-1000° C.
 28. The methodof claim 25, wherein the feedstock has a diameter of one of: (i) betweenabout 5 nm-50 mm; (ii) between about 10 mm-40 mm; and (iii) betweenabout 20 mm-30 mm.